1. Technical Field
The present disclosure relates to a surface-mount antenna and an antenna device including the same.
2. Background Art
Patent Document 1 and Patent Document 2 disclose antennas that operate over a plurality of frequency bands by using a ferroelectric material as a dielectric.
Ferroelectrics have a dielectric constant that changes in response to a voltage applied thereto. The disclosed antennas use this property of ferroelectrics to change the resonant frequency so as to be operable over a wider range of frequencies.
FIG. 1A illustrates a configuration of an antenna disclosed in Patent Document 1. Referring to FIG. 1A, a ground electrode 11 and an inverted-F radiating electrode 12 form an inverted-F antenna, to which power is fed at a feeding point E. At the same time, a ferroelectric component 13 is disposed between an open end of the radiating electrode 12 and the ground electrode 11.
The ferroelectric component 13 disposed between the open end of the radiating electrode 12 and the ground electrode 11 has a dielectric constant that changes in response to a voltage applied thereto. Therefore, the resonant frequency of the antenna provided with the ferroelectric component 13 can be tuned by application of a voltage. However, the antenna suffers high loss because the ferroelectric component is disposed locally at a point of maximum electric field.
FIG. 1B illustrates a configuration of an antenna disclosed in Patent Document 2. The antenna is a so-called patch antenna in which a laminated structure including a ferroelectric layer 23 and paraelectric layers 24 is disposed between a ground electrode 21 and a radiating electrode 22. In this configuration, to change the dielectric constant of the ferroelectric layer by a necessary amount by applying a DC voltage, it is necessary to reduce the thickness of the paraelectric layers. Also in this configuration, to improve the antenna efficiency, it is necessary to reduce the thickness of the ferroelectric layer.
Patent Document 1: PCT Japanese Translation Patent Publication No. 2004-526379
Patent Document 2: PCT Japanese Translation Patent Publication No. 2005-502227
The above-described conventional antennas using ferroelectrics have the following problems to be solved.
(a) Basically, since ferroelectrics typically suffer high loss in high frequency bands, high-gain antennas cannot be obtained. In particular, forming a radiating electrode on the surface of a ferroelectric substrate causes significant gain degradation due to loss resulting from the use of ferroelectrics.
(b) As illustrated in FIG. 1A, when the antenna has a laminated structure of ferroelectric and paraelectric layers and a voltage is applied in the laminating direction, the gain degradation described above can be reduced. However, due to a reduction in the amount of change in capacitance with respect to an applied voltage, a variable frequency range will be narrowed. Therefore, the antenna cannot cover a wide range of frequencies.
(c) In the antennas with conventional configurations illustrated in FIGS. 1A and 1B, when the capacitance between the radiating electrode and the ground electrode is changed by applying a voltage, since the change in capacitance causes a change in impedance, the impedance matching state changes with changes in resonant frequency. That is, the variable range of resonant frequencies in the impedance matching state is narrowed. Thus, it is difficult to achieve impedance matching over a wide range of frequencies.